Synapses undergo a range of plastic changes in structure. Dendritic spines, in particular. display ongoing changes in both shape and number during development, in mature tissue. and in response to changes in their level of activation. The functional consequences of these changes are largely unknown. Hypothesis: Differences in the structure of dendritic spines have a major impact on synaptic efficacy and plasticity. Approach: Although numerous theoretical studies have suggested specific functional consequences of changes in spine shape. the small size of these structures has precluded any direct experimental tests of these hypotheses. We will make innovative use of laser microphotolysis in hippocampal slice cultures to release caged glutamate within a volume corresponding to the size of a single dendritic spine (about 1um). It will thus be possible to stimulate individual dendritic filopodia or spines, record their electrophysiological responses and monitor their morphology simultaneously. Objective: Perform direct tests of the hypothesized relationships between dendritic spine structure and function under several different conditions. Aim 1: Determine the role of postsynaptic glutamate receptor activation in dendritic spine formation. Do filopodia respond electrophysiologically or morphologically to microphotolysis of glutamate? Aim 2: Determine the relationship of dendritic spine shape and postsynaptic responsiveness. How does the shape of a dendritic spine or spine 'morphing' affect electrophysiological responses to glutamate? Aim 3: Determine the consequences of global changes in spine shape on synaptic efficacy and induction of synaptic plasticity. Are responses to synaptically released glutamate or induction of LTP affected by changes in spine shape? Aim 4: Determine the functional consequences of acutely induced changes in spine shape. How do glutamate-induced changes in spine morphology affect electrophysiological responses to microphotolysis of glutamate? Significance: The achievement of these aims will elucidate the fundamental principles underlying the relationship between synaptic structure and function. Furthermore, the results of these experiments will cast new light on the role of structural synaptic plasticity in learning and memory, as well as in neurological diseases of higher cognitive function.